A downhole (borehole) is provided for recovery of hydrocarbon resources (typically called “petroleum” hereinafter) from a subterranean formation containing hydrocarbon resources such as petroleum and gas, but to accelerate the formation and maintenance thereof as well as resource recovery, there are many tools such as frac plugs, bridge plugs, ball sealers, isolation plugs, and packers (comprehensively called “downhole tools” hereinafter) that are disposed of by being disintegrated or dropped in the downhole as-is without being retrieved above ground after use (for examples of such downhole tools and modes of use thereof, see Patent Documents 1 to 6, for example). Therefore, for such disposable tools, it has also been recommended to form the entire tool or a component that constitutes a binding part for accelerating disintegration (component for downhole tool) from a disintegrable polymer. Examples of such disintegrable polymers include polysaccharides such as starch and dextrin; animal protein polymers such as chitin and chitosan; aliphatic polyesters such as polylactic acid (PLA, typically poly-L-lactic acid (PLLA)), polyglycolic acid (PGA), polybutyric acid, and polyvaleric acid; polyamino acids; polyethylene oxide; and the like (Patent Documents 1 and 2). Furthermore, it has also been proposed to pour in a fluid called a pad, such as diesel oil, on top of the frac balls after fracturing using frac balls made from a rigid resin such as polystyrene, to accelerate ball disintegration after fracturing (Patent Document 3).
To recover hydrocarbon resources (typically “petroleum”) from a nearby subterranean formation via a formed downhole, hydraulic fracturing is often employed.
Conventionally, as described above, there were many applications in which a ball sealer as an example of a downhole tool was used in hydraulic fracturing to block perforations directly, as a blocking material (also called perforation balls) for suppressing inflow of excess process water into perforations for recovering petroleum formed using a perforating gun or the like in the subterranean formation (for example, Patent Documents 4 and 5). As ball sealers used in such applications, to improve sealing ability by means of form-fitting deformation into perforations of indeterminate shape as necessary, relatively small ball sealers with a diameter of 16 to 32 mm (0.625 to 1.25 inches; Patent Document 4, column 2, lines 46 to 48) made from a non-disintegrable material such as aluminum or a non-disintegrable resin such as nylon or phenol resin which has been coated with a rubbery surface layer were used. Furthermore, to improve form-fitting deformability into perforations of indeterminate shape, perforation balls having a laminate structure of three or more layers have also been proposed (Patent Document 5).
However, the use of larger-diameter ball sealers as some of the material constituting the frac plug or frac sleeve (plug or pipe for hydraulic fracturing) used in hydraulic fracturing has also been recently proposed. More specifically, a high-pressure water stream is introduced into partitioned process areas by disposing ball seats having an opening at the center, together with frac plugs with incorporated ball sealers for closing the opening, in prescribed locations of the formed downhole, and the water stream is made to act in a direction straight through to the downhole, and the subterranean formation layer is fractured to form perforations for recovering petroleum (for example, Patent Documents 1 to 3).
Alternatively, a method has been proposed wherein a pipe (frac sleeve), in which a plurality of ball seats have been incorporated and disposed with separation therebetween, is inserted into a downhole, and then, in this frac sleeve, a perforation formation operation is continuously performed by successively fracturing the subterranean formation into which ball sealers are supplied to and disposed in the ball seats and then introducing a high-pressure water stream (for example, Patent Documents 6 and 3).
As the ball sealers (also called frac balls) that constitute part of such a frac plug or frac sleeve, in addition to those having the same diameter as the perforation balls used as direct blocking material of the perforation balls generally described above (for example, a diameter of approximately 25 to 100 mm (1 to 4 inches)), those having a larger diameter are often required. Additionally, frac balls require different deformation resistance than perforation balls due to the usage mode thereof. Specifically, in hydraulic fracturing treatment (fracturing) of a subterranean formation, since high water pressure of 7 to 70 MPa (1000 to 10,000 psi) acts on the frac ball, rigidity is required so that breakage or excessive deformation does not occur in order to assure sealability between it and the ball seat. In particular, as shown in Patent Documents 3 and 6, in order to form as many fracturing zones as possible in a sleeve (cylindrical pipe) inserted in a downhole, the difference between the opening diameter of the seat seats that form adjacent seal parts and the diameter of the frac balls must be as small as possible, and the seal width (overlap, difference in radius) between the ball that forms one seal part and the seat must be held to a minimum. Naturally, a frac ball requires deformation resistance (rigidity), which is completely the opposite of the deformability of a perforation ball. For such reasons, conventionally, metal frac balls were mainly used, but it has also been proposed to use resin frac balls to save the labor of retrieval after fracturing (Patent Document 3).
In contrast, the present inventors found that a disintegrable resin frac ball containing, at least in part, an aliphatic polyester resin of which the rigidity (deformation resistance) has been improved by blending a reinforcing material as necessary can be used at least in conventional hydraulic fracturing. It has been established, however, that there are problems in further improving productivity. Specifically, in order to supply and dispose a frac ball of a prescribed size in a ball seat at a corresponding depth reaching 1,000 to 2,000 m from the ground surface, the frac ball must be conveyed over a certain period of time by a high-pressure water stream. This period of time depends completely on the flow rate of the high-pressure water stream, and at a flow rate of not greater than the conventional approximately 4 m/sec (for example, a flow rate of 15 barrels/min for a 4.5-inch pipe), the frac ball containing disintegrable resin described above can be used, but when a higher-rate high-pressure water stream is employed, there is risk that the frac ball will crack and sealing ability will be lost.